Each element of a conventional narrow-spectrum laser device used as the light source of a reduction projection aligner will be described below.
(Exposure Light Source)
With the reduced size and increased integration of semiconductor integrated circuits, demands have been made for improvements in the resolving power of a semiconductor aligner. To respond to these demands, the wavelength of light discharged from an exposure light source has been shortened, and a gas laser device has come to be used as the exposure light source in place of a conventional mercury lamp. A KrF excimer laser device which outputs ultraviolet rays at a wavelength of 248 nm and an ArF excimer laser device which outputs ultraviolet rays at a wavelength of 193 nm are currently in use as exposure gas laser devices. A method of applying an immersion technique, whereby the space between an exposure lens and a wafer is filled with a liquid and the apparent wavelength of the exposure light source is shortened by varying the refractive index, to ArF exposure has been considered as a next-generation exposure technique. In ArF immersion, the apparent wavelength is shortened to 134 nm. An F2 laser device, which discharges ultraviolet rays at a wavelength of 157 nm, is favored as a next-next-generation exposure light source, and the possibility of employing F2 laser immersion exposure exists. It is said that in F2 immersion, the apparent wavelength is shortened to 115 nm.
(Exposure Optical Element and Chromatic Aberration)
A projection optical system is often employed as the optical system of a semiconductor aligner. In a projection optical system, chromatic aberration is corrected by combining optical elements, such as lenses, having different refractive indices. At present, the only optical materials suitable for use as the lens materials of a projection optical system in a laser wavelength region of 248 nm to 115 nm, i.e. the exposure light source, are synthetic fused silica and CaF2. Therefore, a fully refractive monochromatic lens constituted by synthetic fused silica alone is employed as the projection lens of a KrF excimer laser, and a fully refractive partially achromatic lens constituted by synthetic fused silica and CaF2 is employed as the projection lens of an ArF excimer laser. However, the spontaneous amplitude of KrF and ArF excimer laser is wide, between approximately 350 pm and 400 pm, and hence when these projection lenses are used, chromatic aberration occurs, leading to a reduction in the resolving power. It is therefore necessary to narrow the spectral line range of a laser beam discharged from the gas laser device described above to the extent that chromatic aberration becomes negligible. To narrow the spectral line range in a laser device, a spectrum-narrowing module having a spectrum-narrowing element (an etalon, a grating, or the like) is provided in a laser resonator.
(Spectral Purity Range)
The imaging performance of an aligner is greatly affected by the base components of the spectral waveform of the laser beam as well as the full width at half maximum of the spectral waveform. Accordingly, a new index value of the spectrum known as the spectral purity range has been introduced. The spectral purity range is evaluated as a spectral range containing 95% of the entire energy, for example.
To ensure the quality of the integrated circuit, the spectral purity range is preferably held to or below 0.5 pm, for example.
(Reasons for Stabilizing the Spectral Purity Range)
In recent years, however, it has been suggested that the quality of an integrated circuit may deteriorate when the spectral purity range takes a much narrower value than a value for which the optical system is designed. This is described in Patent Literature 1 (U.S. Pat. No. 6,721,340) and Patent Literature 2 (Japanese Patent Application Laid-open No. 2001-267673). It is therefore necessary to control the spectral purity range to be stable within a predetermined allowable range (to be referred to as stabilization control hereafter, where appropriate).
(Related Art of Spectral Purity Range Control)
Patent Literature 1 and Patent Literature 2 relate to spectral purity range stabilization control. Patent Literature 2 describes an invention in which a wavelength detector is provided, a fast tuning mechanism is provided in a spectrum-narrowing unit, and on the basis of the detected wavelength, the wavelength is dithered at high speed and within a narrow range by the fast tuning mechanism at each pulse. As a result, the apparent spectral purity range is controlled within an allowable range. Here, the term “the apparent spectral purity range is controlled” indicates control for artificially obtaining a spectral purity range corresponding to the amplitude by dithering the center wavelength at each moment and time-integrating the results.
Patent Literature 1: U.S. Pat. No. 6,721,340
Patent Literature 2: Japanese Patent Application Laid-open No. 2001-267673